1. Field of the Invention
The present invention relates to an X-ray scanning apparatus of the type associated with computerized tomography.
2. Description of Prior Art
The X-ray scanning apparatus set forth in the preamble is known as an X-ray CT apparatus. It is also known as "third-generation X-ray CT apparatus". In such a third-generation X-ray CT apparatus, fan-shaped X-ray beams are projected toward an object, e.g., a patient, and an X-ray detector having a plurality of detection channels is positioned to receive those fan-shaped X-ray beams which have penetrated through the object and to derive a plurality of X-ray transmission signals therefrom during the revolution examination. A data acquisition unit employing a multiplexer and an analogue-to-digital (A/D) converter is provided in which a plurality of analogue X-ray transmission signals are transferred through the multiplexer to the A/D converter so as to convert the analogue X-ray transmission signals into digital transmission signals. A reconstruction device is provided to process the digital transmission signals from the A/D converter so as to reconstruct a computerized tomographic image of the object. In general, the sequence of analogue-to-digital conversion is performed from "a central channel region" to "a peripheral channel region" of the X-ray detector. It should be noted in the present specification that the above-described "central channel region" of the detection channels of the X-ray detector is defined as the detection channels located around the center of the plurality of detection channels, and the "peripheral channel region" is defined as the detection channels located near both ends of the plurality of detection channels.
A main reason why such an A/D conversion sequence has been employed in conventional X-ray scanning apparatus is that the longer a time period becomes during which the X-ray transmission signals are detected and thereafter A/D-converted via the multiplexer, the greater are the noise signals mixed with the A/D-converted transmission signals. In addition to the above-described fact, it is known that the detected transmission signal, or detection signal which is derived from the central channel region causes an artifact, as compared with the detection signal derived from the peripheral channel region. Accordingly, the total amount of noise signals of the detection signal is reduced by A/D-converting the X-ray transmission signal that is derived from the central channel region prior to one derived from the peripheral channel region. That the resultant tomographic image is not to any extent adversely effected by the artifact
There is another proposal for increasing the A/D conversion rate by parallel operations of a plurality of A/D converters.
However, the conventional X-ray scanning apparatus owns the other problems. That is, when the detection signals from the central channel region are A/D-converted at the beginning of the A/D conversion sequence, the slewing rate of the operational amplifiers in the data acquisition unit must be suppressed to a lower value, because a charging current flows through filters in the data acquisition unit for the sake of noise reduction.
The input terminals of the A/D converters, on other hand, are kept to the ground potential when no input signal is applied to those input terminals. Usually the detection signals which are to be A/D-converted at the beginning, have rather large amplitudes. Accordingly, if, as described above, the slewing rate is suppressed to a lower value, the A/D converters cannot sufficiently follow such large input signals. As a result, when the above-described sequence of A/D conversion from the central channel region to the peripheral channel region is employed, artifacts may occur in the resultant tomographic image of the conventional X-ray scanning apparatus.
Further, in a data acquisition unit having a plurality of A/D converters, the central channel region of the respective detector is normally divided into two subregions, taking acount the design limitation for the detector and the multiplexers that receive the X-ray transmission signals and are coupled to the A/D converters. Since, for example, two A/D converters whose characteristics are different from each other are coupled to the subregions of the detector, a transition consequently exists in the A/D-converted transmission signals with respect to the central channel region, so that an artifact appears in a portion of the tomographic image which corresponds to the central channel region. Accordingly, great attention has been paid only to an improvement of the characteristics of the A/D converters.
It is therefore an object to provide an X-ray scanning apparatus in which the tomographic images having no artifact can be obtained by introducing a new sequence of A/D conversion, taking account inherent problems of a data acquisition unit having a plurality of A/D converters. Those inherent problems are known as differences in the characteristics of the A/D converters such as the linearity, offset value and so on.